Angiotensin II antagonists are medicaments useful in the treatment of hypertension, anxiety, glaucoma and cardiac attacks. A number of these compounds share a biphenyltetrazole moiety and can be represented by the following formula (I)

wherein Z is an optionally substituted heterocycle containing at least one nitrogen atom; or an open amide residue.
More particularly, the substituent Z can have the following meanings

which identify specific angiotensin II antagonists: (a) [2-butyl-4-chloro-5-hydroxymethyl-imidazol-1-yl] losartan; (b) [2-ethoxy-3H-benzimidazole-4-carboxylic acid] candesartan; (c) [2-butyl-1,3-diaza-spiro[4,4]non-1-en-4-on-3-yl] irbesartan; (d) [(S)—N-(1-carboxy-2-methylprop-1-yl)-N-pentanoylamino] valsartan; and (e) [5-carboxy-4-(1-hydroxy-1-methylethyl)-2-propy-imidazo-1-yl] olmesartan.
A number of processes for the preparation of the compounds of formula (I) are known. For example, according to EP 253310, losartan can be prepared by reaction between a compound of formula (III)

wherein X is a leaving group and P is a tetrazole protecting group, and a base Z1-H, wherein Z1 has the meaning as indicated in (a); the subsequent removal of the protecting group P affords losartan.
WO93/10106 discloses the preparation of losartan by cross coupling reaction between a compound of formula (IV)

wherein Z1 has the meaning as indicated in (a) above, Y is a leaving group such as halogen, methanesulfonyloxy, fluorosulfonyloxy or trifluoromethanesulfonyloxy; and a synthon of formula (V)

wherein —B(R1R2) is a disubstituted boron atom and P is a tetrazole protecting group, to obtain the compound of formula (II) reported above, and subsequent removal of the protecting group P.
Candesartan, irbesartan, valsartan, and olmesartan can be obtained analogously.
Generally speaking, a protecting group is a group which blocks a reactive site in a compound having more than one reactive group, so that the chemical reaction can be carried out selectively on another unprotected reactive site, said protecting group being easily removed at the end of the selective reaction.
The compounds of formula (II), (III) and (V) reported above contain a tetrazol-5-yl group, in which a reactive nitrogen atom is present. It is known that the reactive tetrazole nitrogen in a compound of formula (II), (III) and (V) can be protected with a group such as triphenylmethyl, p-nitrophenyl, pyridyl or pyrimidyl. Analogously, the 1-methyl-1-phenylethyl group would be a very good protecting group for the reactive tetrazole nitrogen, in that it is particularly resistant to cleavage during the selective reaction, but it also is resistant to cleavage after completion of the reaction.
U.S. Pat. No. 5,412,102 discloses the use of the 1-methyl-1-phenylethyl group (cumyl) as a protecting group in the preparation of 1-butyl-2-[2′-(2H-tetrazol-5-yl)biphenyl-4-yl-methyl]-1H-indole-3-carboxylic acid. The deprotection is carried out by reaction with about 3-4 equivalents of a Lewis acid, preferably boron trifluoride etherate (see example 15), in the presence of a thiol, an arylthiol, a 2-mercaptoacetic acid, preferably a 2-mercaptoacetate, or pentaerythritol tetrakis(2-mercaptoacetate).
The removal of the cumyl protecting group is efficient, but the reagents used do not fulfil the requirements for the production on an industrial scale. Boron trifluoride etherate is in fact a dangerous, flammable compound (Bretherick, Handbook of reactive chemical hazards, IV ed. page 481). Similarly, thiols and the derivatives thereof are remarkably difficult to use. There is therefore the need for of a novel process which allows to remove the 1-methyl-1-phenylethyl group from the phenyltetrazole derivatives, overcoming the mentioned problems.